1. Field of the Invention
This invention relates to hydraulic or pneumatic valves having piezo-electric actuators, and in particular to such valves having a piezo-electric actuator in the form of a thin sheet made up of a piezo lamina and an electrode lamina in which edges of the sheet are constrained against axial motion by the valve housing and which bows or cups in the middle when electrically excited.
2. Discussion of the Prior Art
Piezo-electric materials are well known and have been used in a variety of applications. A material possesses piezo-electric properties if it produces an electric charge when it is subjected to a mechanical stress. When so subjected, a piezo-electric material exhibits what is known as the "generator effect". Conversely, when an electric field is applied to a piezo-electric material, a mechanical stress is induced in the material which causes it to deflect. This phenomenon is referred to as the "motor effect".
Some piezo-electric materials are naturally occurring crystalline materials such as quartz and tourmaline. Artificially produced piezo-electric crystals include rochelle salt, ammonium, dihydrogen phosphate (ADP) and lithium sulphate. Piezo-electric materials also include polarized piezo-electric ceramics. Piezo-electric ceramics include lead zirconate titanate, barium titanate, lead titanate and lead metaniobate.
Various processes are involved in the production of piezo-electric ceramics. Typically, the production process includes mixing the raw materials, heating the powders so as to react the constituents into a compound (known as "calcining") and grinding the calcined powders. Various shapes may then be formed of the powders using a binder to hold the shapes together prior to firing. A number of firing steps may then occur to burn off the binders and provide mechanical strength, and at higher temperatures to chemically bond the material together. Electrodes are then applied to the desired surface or surfaces of the piezo-electric ceramic part. The electrode may take the form of a layer of electrically conductive material such as silver oxide which is chemically deposited on the surface of the piezo-electric ceramic in a coating process, or maybe an electrically conductive sheet such as invar, copper, nickel, brass or silver which is laminated to the surface of the piezo-electric ceramic.
The piezo-electric properties are activated in the manufacturing process when the part with the electrodes applied is subjected to a high electric field so as to align the dipoles within the ceramic material. Aging of the ceramic then occurs in which the dipoles relax and eventually reach a steady state. Depolarization of the piezo-electric ceramic can result from excessive heat, electrical or mechanical stress or combinations of these conditions.
Piezo-electric ceramics have found usage in many different applications. For example, piezo-electric ceramics are used in sonar and specialized transducers, in fish finders, in ultrasonic applications such as cleaners and other cavitation products, in high sensitivity hydrophones and other receiving devices, in accelerometers, and in electro-acoustic devices.
Piezo-electric ceramics have also been applied to actuate hydraulic and pneumatic valves. In known valve actuators, thin sections of piezo-ceramic material have been stacked between multiple electrodes so as to produce a column of alternating layers of piezo material and electrode material. Individual voltages are then applied to the electrodes so that each layer of piezo-ceramic material changes slightly in thickness, but when all the changes in thickness add up due to the mechanical relationship of the piezo ceramic layers to one another, the total deflection of the column is sufficient to modulate the opening and closing of a hydraulic valve. While the force output is high and the displacement can be made sufficient, these types of actuators are bulky and expensive due to the volume of piezo ceramic material required.
In another application of piezo ceramic material to actuate hydraulic valve, sometimes referred to as a "piezo bender", a strip of piezo ceramic material with an electrode laminated to one side is cantilevered at one end and at the opposite end overlies the seat of the hydraulic valve. The strip can be caused to bend toward or away from the seat, depending upon the magnitude and polarity of the voltage applied across the piezo ceramic material. A variation on this is laminating two oppositely poled piezo strips to one another, rather than one piezo strip to a metal electrode. Relatively large displacements (for example 0.010") can be obtained, but the force output and the stiffness of the strips are very low.
Thin sheets of piezo-electric ceramics laminated to an electrode sheet are known. When a layer of piezo-electric ceramic is laminated to only one side of the electrode sheet, the composite is known as a monomorph. When the electrode has a layer of piezo-electric ceramic material laminated on both of its sides, the resulting laminate is known as a bimorph. When excited with an electric field and constrained along opposite edges, a monomorph or bimorph deflects into a bowed shape if it is relatively long and wide or a cupped shape if it is both long and wide.
The amount of deflection of a monomorph or bimorph depends upon the voltage applied, within limits. When the voltage is oscillated, the monomorph or bimorph also oscillates, and can be oscillated at a relatively high frequency, for example 300.sub.hz. Perhaps the most common application of monomorphs and bimorphs has been in acoustic devices, in which the monomorph or bimorph has been oscillated to produce a sound, much like the cone of a conventional speaker is oscillated to produce sounds.
Solenoid operated valves in which an armature is opened or closed against a nozzle type valve seat are also known, for example from U.S. Pat. Nos. 4,774,976 and 5,328,147. The solenoids associated with operating these valves are relatively heavy, large and expensive. In addition, the mass of the armatures and the coil inductance limit the frequency at which they can be oscillated, for example if the solenoid is energized with a pulse width modulated electrical signal. The limitation of the frequency can cause the resulting pressure to be uneven, displaying pressure dither, and also limits the control pressure band width.